By Martin Stevens
Our world is dominated by colours and patterns that provide information about how to behave and survive. These are a product of how our sensory system and brain interpret the physical properties of the environment. For example, how people see and describe colours can depend on whether they have ‘normal’ colour vision or not, what culture they come from, and even what their emotional state is. Colour is in the eye of the beholder!
However, the differences in perception among humans are a drop in the ocean compared to how other animals view the world. Even animals with the same sensory modalities (e.g. vision) have substantial differences across species. Take vision and ultraviolet (UV) light. We can’t see UV because our lens blocks these very short wavelengths before they can reach the photoreceptors. In contrast, many animals detect and use UV for a variety of tasks. For example, flowers may look beautiful to us, but seen through the eyes of a bee they take on a whole new appearance. Many petals have lines that are only visible in UV that run towards the centre of the flower. These ‘nectar guides’ act like landing lights, directing insects to the pollen and nectar. Birds also use UV. Many of us will have seen kestrels hovering at the side of a road or field. They can detect the presence of voles by picking up UV light that’s reflected by vole urine along trails. Blue tits provide another example. They look identical to us but the male plumage reflects more UV than females, and males with greater UV are more attractive in mating.
Ultraviolet light is just one example of differences in vision among animals. Some of the most extreme cases come from the deep ocean. Longer (‘red’) wavelengths of light fail to reach here, making the conditions blue (rich in short wavelengths). In addition, almost all bioluminescence in the ocean is blue or blue-green, and because it doesn’t make sense to have a visual system that detects light that isn’t there, most animals in the depths can’t detect red light. Dragon fish, however, produce red bioluminescence and have a visual system to detect it. This means that they can signal to each other without attracting unwanted attention, and even use their red flashlights to hunt without the prey knowing that they’re there!
So far, we’ve focussed on vision. But humans have other senses too – touch, taste, smell, and hearing. Unfortunately, there’s nothing exceptional about any these either. We can hear sounds roughly in the range of 15-20,000 Hz (20 kHz), but this fails to match many species. For example, bats navigate and detect prey using ultrasonic echolocation (sometimes well above 100 kHz), which can be so refined that they can determine the size, texture, speed, and even wing beat frequency of their insect prey! Ultrasonic sounds are used by other animals too, such as mice and rats in aggressive encounters, mate attraction, and even to beg for food.
Animals also differ in what sensory systems they have, with humans lacking electric and magnetic senses that are found in other species. Birds are particularly well studied for how they use the earth’s magnetic field to navigate. Bees and other insects can also orientate based on magnetic cues, and as if echolocation wasn’t enough, so too can some bats. Equally fascinating is the use of electric information. Sharks and rays, and even some ‘primitive’ mammals like platypus and echidna can detect the electric fields generated by other animals (usually prey). Some fish from South America and Africa can even produce electricity and use this to navigate, find prey, identify their own species, and assess potential mates. Just as male and female blue tits look different to bird eyes, many electric fish have different electric signal forms for males and females.
What drives differences among animal sensory systems? We don’t have all the answers to this important question but it’s clear that the environment that an animal evolved and lives in plays a major role. For example, electricity isn’t well conducted by air and so an electric sense would be useless to most terrestrial animals. Many fish with an electric sense live in murky disturbed waters or are nocturnal. Here, vision is more limited so they use other senses. Animals have also evolved sensory systems that are tuned to specific tasks. For example, some primates, including humans, evolved the ability to see differences between reds and greens (which most mammals don’t) for finding ripe fruit against green leaves. Finally, sensory systems take up significant resources and energy. There’s simply no reason to invest in a sensory system that’s not useful – natural selection doesn’t like waste! It’s why cave animals like blind cavefish lose their visual systems when they move into dark environments.
These are exciting times to study the perceptual worlds of animals. New technologies have made it possible to eavesdrop on once hidden modes of communication and to appreciate how other species interpret their environment. Above all, we are obtaining a much better picture of how sensory systems shape animals’ lives and the role they play in evolution, and even how we can learn from them to improve our own lives.
Martin Stevens is a BBSRC Senior Research Fellow, based in the Centre for Ecology & Conservation, University of Exeter. His research focuses on sensory ecology and behaviour, especially animal coloration and vision, across a range of organisms. It covers bird vision, anti-predator defences such as camouflage and warning signals, brood parasitism and cuckoos, and sexual signals and vision in primates. He is the author of Sensory Ecology, Behaviour, and Evolution (OUP, 2013). Find Martin Stevens on Twitter: @SensoryEcology.